1. Field of the Invention
The present invention relates to a film forming method and a film forming apparatus suitable for forming a film through a thermal CVD (Chemical Vapor Deposition) process.
2. Description of the Related Art
In forming a wiring film such as a polysilicon (Poly-Si) film or a tungsten (W) film, on a semiconductor wafer (hereinafter simply referred to as wafer) through a thermal CVD process, remove of an SiO.sub.2 film on the wafer surface is wanted in order to provide good electric contact between the polysilicon film and its base film.
For example, as shown in FIG. 13, an SiO.sub.2 film 2 is formed on the surface of a P type silicon substrate 1. Then, a predetermined region of this SiO.sub.2 film 2 is etched off. An N.sup.+ region 3 is formed in that portion of the substrate 1 which lies directly under the etched region. Then, a polysilicon film 4 is formed on the SiO.sub.2 film 2 through thermal CVD so as to cover the N.sup.+ region.
In this case, exposing the substrate 1 to air prior to forming of the polysilicon film 4 forms an SiO.sub.2 film 5 in the N.sup.+ region through natural oxidization; this film 5 is an insulating film. If the polysilicon film 4 is formed under this condition, therefore, it is likely to provide poor electric contact between the polysilicon film 4 and the N.sup.+ region 3 as the base film. Since the contact area between the polysilicon film 4 and the N.sup.+ region 3 becomes relatively smaller with an increase in integration density of a semiconductor device, the problem of this proper electric connection at the contact portion becomes more significant.
Conventionally, the polysilicon film 4 is formed after removing the SiO.sub.2 film 5 through wet cleaning using a cleaning solution.
The wet cleaning of the substrate, however, is very troublesome in views of manufacturing process and requires a significant amount of time and labors in fabrication of a semiconductor device.
In addition, inside a thermal CVD device is always heated at 600.degree. to 800.degree. C., so that when the substrate is put in the thermal CVD device, an oxidization film is formed about 10 to 30 .ANG. thick on the surface of the substrate by natural oxidization with the irradiated heat and oxygen in the air. Even if the wet cleaning is performed in advance, therefore, when the substrate 1 is transferred in a reaction container of the thermal CVD device, an oxidization film 5 is again formed on the surface of the substrate 1 through natural oxidization. As a result, the SiO.sub.2 film 5 originated from the natural oxidization cannot be completely removed. It is of course possible to prevent the SiO.sub.2 film from being formed through natural oxidization at the time the substrate is inserted into the reaction container if the temperature of the thermal CVD device is reduced below 300.degree. C. However, it takes a considerable time to increase or decrease the temperature of the thermal CVD device. In this respect, the alteration of the temperature of the thermal CVD device would certainly reduce the fabrication efficiency significantly.
To process a LCD (Liquid Crystal Device) substrate, a semiconductor substrate, or the like, there is a device which forms a passivation film such as an SiO.sub.3 N.sub.4 film on a substrate wafer after Al wiring, for example. This type of film forming apparatus forms a film at as low a temperature as 400.degree. C. or below, for example.
FIG. 14 illustrates a conventional plasma CVD device for forming such a passivation film. In a cylindrical reaction container 11 made of quartz are disposed a pair of comb shape high frequency electrodes 13 coupled to a high frequency power source 12, with their electrode plates 14 (comb portions) placed alternately. An object to be processed (hereinafter referred to as target object), such as a semiconductor wafer 15, is adhered to either side of each etching gas plate 14.
A gas inlet 17 for introducing a reactive gas 16 consisting of, for example, an SiH.sub.4 +NH.sub.3 gas is provided at one end portion of the reaction container 11. A heater mechanism 18 is provided around the reaction container 11.
A passivation film is formed by such a plasma CVD device as follows. First, the processing temperature inside the reaction container is set to a predetermined level by the heater mechanism 18. Then, a predetermined reactive gas 16 is introduced through the gas inlet 17. A Si.sub.3 N.sub.4 film is formed while transforming this reactive gas 16 into plasma between the high frequency electrode plates 14. At this time, the inside of the reaction container 11 is deaired through an air outlet provided at the reaction container by a vacuum mechanism 10 so as to provide a predetermined vacuum level in the reaction container 11.
According to this plasma CVD device, however, since a target object is located close to where plasma is generated, it is likely to be damaged by the generated plasma. In addition, a reaction-originated film adhered to each electrode plate 14 is separated by the plasma to be dust which is in turn undesirably adhered to the target object. Since the high frequency electrodes 13 themselves have a heat capacitance, it is not possible to quickly increase the temperature inside the reaction container 11 to a predetermined level. Further, the plasma CVD device has a poor response to temperature control, making it difficult to ensure a temperature control with a high accuracy.
As a solution to these problems, there has been proposed a so-called active gas transporting system which produces plasma outside the reaction container 11, then sends it inside this container 11 for the necessary treatment (refer to the Japanese Journal of Applied Physics, Vol. 117 (1978), Supplement 17-1, pp. 215-221). FIG. 15 illustrates a plasma CVD device utilizing this active gas transporting system. Disposed in a substantially spherical reaction container made of quartz is a heating table 23 on which a semiconductor wafer 22 as a target object is placed to be heated. A pair of process gas inlets 25 for introducing a process gas such as an N.sub.2 or O.sub.2 gas are provided at respective end portions of the reaction container 21. A reactive gas inlet 27 for introducing a reactive gas such as SiH.sub.4 is provided at the upper portion of the reaction container 21, and an air outlet 29 is provided at the lower portion. This air outlet 29 is connected to a vacuum mechanism 29 for maintaining the inside of the reaction container 21 at a predetermined vacuum level.
Provided outside the reaction container 21 is a plasma generating mechanism 33 which comprises a plasma generating container 30 for introducing a process gas 24, a microwave guide 31 and a microwave output section 32.
According to thus constituted plasma processing device, a process gas such as an N.sub.2 or O.sub.2 gas transformed into plasma by the plasma generating mechanism 33 is introduced via a transport tube 34 in the reaction container 21 from the process gas inlet 25. A Si.sub.3 N.sub.4 film is formed under the atmosphere of such a mixed gas at, for example, 400.degree. C.
The prior art is intended to overcome the afore-mentioned problem of damaging a target object or generating dust by providing the plasma producing section outside the reaction container.
According to the conventional plasma processing device utilizing the active gas transporting system, however, since a target object is directly heated on the heating table, it cannot be uniformly heated. This is likely to provide an uneven treatment. Further, the reactive gas 26 and the process gas 24, introduced in the reaction container 21, are not uniformly mixed, which is likely to cause a variation in density distribution of these gases depending on which portion in the reaction container 21. This also stands in the way of providing an even treatment. These problems become more prominent as the size of the reaction container 21 increases. Further, a plurality of heating tables is necessary for treating a number of objects at the same time. This prevents enlarging of the reaction container 21 for processing a large quantity of target objects at a time, and thus makes the prior art disadvantageous for mass production.